The file system depends on a number of special areas of the disk set aside for organization when the disk is formatted: the master boot record, the partition table, the boot record, the file allocation table (from which the FAT system takes its name), and the root directory. At a low level, disks are organized into 512-byte groupings called sectors.
The FAT system allocates space for files using a unit called a cluster, made up of an integral number of sectors. The number of sectors in a cluster must be a power of 2. (You can run CHKDSK or SCANDISK to see how big the clusters are on your system.) Microsoft calls these clusters allocation units, and SCANDISK's report includes a notation of their size, such as "16,384 bytes in each allocation unit." You can normally compute the cluster size by dividing the disk size by 64K (65,536 bytes) and rounding up to the nearest power of 2. Thus, a 1.2GB disk has clusters that are 32K in size: 1.2GB (1,258,291.2K) divided by 65,536 is 19.2K, which rounds up to 32K.